The present invention relates to a silo for storing particulate material, e.g., but not limited to, hot asphaltic mix, aggregates, cement and concrete mixtures and particularly relates to a storage and discharge system for discharging the material from the silo into trucks.
Silos for receiving, storing and discharging particulate materials, for example, hot asphaltic mix, are typically employed before delivery of the materials to an end user for a number of different reasons. Batch asphalt plants utilize storage silos for temporarily storing the asphaltic mix, and this enables the plant to run at a more consistent rate, which effectively improves plant productivity. For drum mix plants where the product, e.g., hot asphalt mix, is made continuously, silos effectively provide for additional surge capacity. Thus, silos enable the drum mix plant to operate continuously to produce the asphaltic mix while trucks deliver the asphaltic mix on a batch basis. Additionally, multiple silos are used to make available different types of mixes without having to switch mix designs on the production units. Also, since a mix is already made and can be dispensed as needed from a silo, trucks can be loaded with the asphaltic mix from the silo much faster for delivery to the end user and can increase the production capability of a particular plant. Further, silos enable the storage of the material for limited periods of time such as overnight so that the mix will be available early in the morning.
Storage silos, however, introduce several problems between the production of the mix and its delivery to an end user. Segregation of the mix, i.e., separation of larger aggregate from smaller aggregate in the mix, can occur in the silo due to flow differences between different sized aggregates. This may result in providing an end user with a non-uniform mix. Further, current single outlet silos may produce tunneling, i.e., “rat holing,” of the mix flowing through the silo. That is, the material in the center of the silo may discharge along with the material above it leaving the material closer to the outer walls stagnant. This can contribute to the problem of segregation as well as other problems. Also, when storing asphaltic mix in a silo, oxygen can migrate into the silo and oxidize the mix creating hardened aggregate chunks, which will plug the silo and/or cause quality issues when using the asphaltic mix in a paving operation.
The problem of segregation of the aggregate is also influenced by the speed in which each truck is loaded with mix from the silo. For example, when a truck is located in the loading area under the silo with a single discharge outlet, and not moved during loading, the discharge provides a mix sensitive to segregation. That is, larger stones may roll to the front, to the rear and to each side of the truck leaving coarse material about the periphery of the pile and a fine mix in the middle of the pile loaded onto the truck bed. This coarse material then is the first and last material to be discharged from the truck bed. The coarse material may then be trapped in the wings of the paver resulting in coarse areas of pavement between each load. To prevent this type of segregation during loading, it is common practice in current single outlet silos to provide three separate discharges from the silo into each truck. For example, the first discharge may be made directly adjacent the front of the truck bed, the second discharge directly adjacent the tailgate and the third discharge in the center. By using this method, coarse material is forced to roll to the middle of the truck bed and then is covered up by the last discharge, thereby rendering greater uniformity of the aggregate material when discharged from the truck. These discrete steps and movements of the truck, however, slow the loading process although they provide some assurance that the load dumped from the truck will be substantially uniform with the coarse material intermixed. Thus, there has developed a need for a system for discharging particulate material from storage silos that will avoid the foregoing and other problems associated with prior silo discharge systems.